1. Field of the Invention
The present invention relates to a surface acoustic wave device, and more particularly, to a surface acoustic wave device that is especially useful as a band filter or a resonator.
2. Description of the Related Art
Conventionally, surface acoustic wave devices are widely used as a band filter and a resonator. For surface acoustic wave devices to be used in these fields, it is strongly required to have a good frequency characteristic. In the case in which an interdigital transducer (hereinafter, referred to as IDT), and a reflector are arranged as a film on a surface acoustic wave device, the larger the film thickness is, the longer the film-forming time is, and moreover, it is more difficult to obtain a uniform film thickness. Thus, it is desired that the film thicknesses of the IDT and the reflector are small.
Accordingly, in surface acoustic wave devices using an shear horizontal or SH wave, metals having a high density such as Au, W, Ta, Pt, and so forth are used for the IDTs and the reflectors in many cases. When a metal material having a large density such as Au is used for the IDTs and the reflectors, which have a small film thickness, can easily excite and reflect an SH wave. Thus, the thicknesses of the IDTs and the reflectors can be reduced.
Moreover, when plural surface acoustic wave devices (especially, surface acoustic wave devices for use as narrow-band filters) are produced, it is desired that dispersions in center frequency between the respective surface acoustic wave devices are as small as possible. Accordingly, regarding surface acoustic wave devices having IDTs and reflectors using metals with a high density such as Au, W, Ta, Pt, dispersions in frequency between the respective surface acoustic wave devices in the same wafer are suppressed by making the film-thicknesses of the IDTs and the reflectors as uniform as possible when the IDTs and the reflectors are film-formed.
However, techniques for making uniform the thicknesses of the IDTs and the reflectors have a limitation. Therefore, when a plurality of surface acoustic wave devices are produced on the same wafer at one time, dispersions in frequency between the respective surface acoustic wave devices become large. Therefore, even if the metal materials having a high density such as Au, W, Ta, Pt, are used, practically it is necessary to adjust the frequencies of the finished surface acoustic wave devices, individually.
Thus, though it is assumed that the metal materials having a high density such as Au, W, Ta, Pt, are desirably used to produce the IDTs and the reflectors of the surface acoustic wave devices using an SH wave, it is necessary to adjust the frequencies individually. Thus, the throughput is reduced, which increases the cost.
As a method of adjusting the frequencies of the surface acoustic wave devices, individually, a method of etching the surfaces of the IDTs and the reflectors by use of ion beams, a method of forming films as insulators between substrates, the IDTs, and the reflectors, a method of etching the substrates, the IDTs, and the reflectors according to RIE (reaction ion etching), and so forth, are generally used. For this reason, when the frequencies are adjusted by use of ion beams or the like, the IDTs, the reflectors, and the substrates suffer damage, which deteriorate the characteristics of the surface acoustic wave devices.
When plural surface acoustic wave devices are produced using the same wafer, dispersions in center frequency can be reduced by use of metal materials having a high density such as Ni, Al, Cr, Cu, for the IDTs and the reflectors. However, the metal materials such as Ni, Al, are difficult to excite and reflect an SH wave. Thus, filters and resonators having a good frequency characteristic can be obtained but with great difficulty. The metal materials are not suitable as materials for the IDTs and the reflectors.
In order to solve the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device utilizing an SH wave which minimizes dispersions in center frequency, so that it is not necessary to adjust the frequency after the IDT and the reflector are produced.
The surface acoustic wave device according to a preferred embodiment of the present invention utilizes excitation of an SH wave, and includes an interdigital transducer or a reflector defined by a laminated body including at least total three metal layers having at least one first layer made of a metal with a density of at least about 15 g/cm3 as a major component and at least one second layer made of a metal with a density of up to about 12 g/cm3 on a piezoelectric substrate, the volume of said first layer being in the range from about 20% to about 95% of the total volume of the interdigital transducer or the reflector. The first and second layers are preferably formed, e.g., by vapor deposition or sputtering.
The surface acoustic wave device according to preferred embodiments of the present invention is preferably used as a filter, a resonator or other electronic component utilizing an SH wave. The second-layer containing a metal with a density of up to about 12 g/cm3 such as Ni, Cr, Cu, Al, Ti, or other suitable material is sandwiched between the first layers containing as a major component a metal with a density of at least about 15 g/cm3 such as Au, W, Ta, Pt, or other suitable material. The second layer is a metal layer having a small effect of reducing the propagation velocity of a surface acoustic wave on the piezoelectric substrate. Since the second layer is sandwiched between the first layers, dispersions in frequency of the IDT or the reflector, caused by dispersions in film thickness thereof, are minimized. Thus, excellent resonator and filter characteristics are achieved.
Accordingly, in the case in which a plurality of surface acoustic wave devices are formed on the same wafer, frequency adjustment of the individual surface acoustic wave devices is unnecessary. The cost of the surface acoustic wave device can be reduced, due to the enhanced throughput. Moreover, the frequency adjustment by ion beam etching or other suitable process is not required. Thus, damage to the piezoelectric substrate, the IDT, and the reflector is prevented, and moreover, the acceptance ratio of the surface acoustic wave devices is greatly improved.
The volume of the first layer is preferably in the range of from about 20% to 95% of the overall volume of the IDT or the reflector. To decrease the film-thickness of the IDT or the reflector, desirably, the ratio of the first layer is high. For this reason, preferably, at least two first-layers are contained in the IDT or the reflector.
In the surface acoustic wave device of preferred embodiments of the present invention, preferably, in the layers lying in the range of the thickness of up to one-fourth of the total thickness of the interdigital transducer or the reflector measured from the surface of the piezoelectric substrate of the metal layers constituting the interdigital transducer or the reflector, the first-layer has a volume of at least about 50%. Moreover, preferably, the metal layer disposed directly on the piezoelectric substrate is the first layer or the second layer which has a small thickness. That is, preferably, of the layers each having a thickness of at least about one-twentieth of the total thickness of the interdigital transducer or the reflector, the layer located nearest to the piezoelectric substrate is the first-layer. Moreover, preferably, the surface of the IDT or the reflector includes the first layer.
In particular, according to another preferred embodiment of the present invention, preferably, the first layer contains Au as a major component and has a volume of from about 40% to about 80% of the total volume, and the second layer contains Ni as a major component and has a volume of from about 20% to about 60% of the overall volume.
According to another preferred embodiment of the present invention, the first layer contains Au as a major component and has a volume of from about 20% to about 50% of the overall volume, and the second layer contains Al as a major component and has a volume of from about 50% to about 80%.
According to preferred embodiments of the present invention, at least two of the first layers with a high density having a high effect of decreasing the propagation velocity of a surface acoustic wave on a piezoelectric substrate, and the second layer with a low density having a small effect of decreasing the propagation velocity of the surface acoustic wave on the piezoelectric substrate, that is, being capable of decreasing dispersions in frequency caused by dispersions in film thickness are arranged so as to have a laminated structure. Thereby, excellent resonator and filter characteristics are obtained. Moreover, dispersions in center frequency between plural surface acoustic wave devices produced in the same wafer are minimized.
Accordingly, it becomes unnecessary to carry out the frequency adjustment individually, which has been conventionally needed. Significant improvement in throughput can be realized. Furthermore, cost reduction can be achieved. Moreover, damage and sticking of foreign matters to the piezoelectric substrate caused by frequency adjustment can be eliminated. This is advantageous from the standpoint of characteristics. The acceptance ratio is greatly increased.
Other features, elements, characteristics and advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.